ESTRO 2025 - Abstract Book

S4365

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Belt™ system. Targets within the myocardium were delineated jointly in Eclipse TPS by an oncologist and cardiologist using the AC scan for treatment. Volumes were contoured retrospectively on the NAC scans for direct comparison with AC volumes. Target volumes and stomach/PTV overlap were compared. Results: For nine out of ten patients, the PTV volume was smaller with AC compared to NAC (mean reduction 7.7±24.4cc, p=0.078). GTV and ITV volumes were also smaller with AC but to a lesser extent and for only 7 patients (mean reduction 2.3cc, p=0.237 and 2.2cc, p=0.469 respectively). Four out of ten patients had a non-zero stomach/PTV overlap in both AC and NAC. NAC reduced overlap in 5 patients and increased in one (mean change -0.4±5.2cc).

Conclusion: This study shows a potential benefit of using AC in cSABR in terms of an approximately 10% PTV volume reduction compared to NAC. However, due to the small cohort size of our study this comparison did not reach statistical significance (p=0.078). AC did not seem to offer any benefits in terms of the overlap volume between the PTV and the stomach which is the main organ at risk in this group of patients. The effect of AC on the position of other OARs such as bowel, chest wall and coronary arteries is yet to be investigated. As we are building our local and national cSABR experience in the UK, a larger cohort will enable us to investigate these volumetric results as well as their dosimetric effects.

Keywords: CSABR, Abdominal Compression, Scans

2758

Digital Poster Dosimetric Evaluation of Superficial Parotid Lobe Sparing IMRT Techniques for Nasopharyngeal Carcinoma: Insights on Feasibility and Trade-offs Pui Lam Yip, Shao Bin Cai, Tabitha Chan, Shing Fung Lee, Timothy Cheo Radiation Oncology, National University Cancer Institute, Singapore, Singapore